I think the biggest mistake you can make during the first 6 to 12 months of training is not focusing on getting stronger. There are no weak, muscular men. This is not to say you have to try and "max out", or train with low reps. Not so.

Use compound exercises and work mainly between the 8-12 rep range for the first 6 months. After that point really anything around the 5-12 reps will serve you well. You want a few more reps early on to practice form, build stability, etc.

Get stronger in these rep ranges, and with compound exercises.

The next big mistake is thinking that muscle building is complicated. It's not. You don't need massive volume during the first 6-12 months. Forget about drop sets, super sets, pumping, priming, pre-exhaust, Doggcrapp, slow negatives and elaborate 5-6 day splits.

These may become useful tools down the road, but they fairly useless if you are still relatively weak.

Another common mistake is buying into muscle confusion. Changes will be needed at some point down the road to challenge your body. But right now the only thing needed to confuse you muscles is more weight on the bar.

Stop changing programs. Find something you like and stick with it. There are no magic programs. Progression of weight is the magic.